The present invention relates generally to methods and scanning beam systems that provide high frame rates. More particularly, the present invention provides methods and systems for removing energy that is stored in a resonant scanning element.
A scanning beam device that has been developed by the University of Washington uses a single optical fiber to scan an illumination spot over a target area in a one or two dimensional scan pattern. Light reflected from the target area is sequentially captured by a light detector, such as a photodetector. The photodetector response is then used to determine the brightness of the small portion of the image that corresponds to the small area illuminated by the illumination spot at that given point in time during the scanning pattern.
While the optical fiber can be driven at any number of frequencies and in any number of one or two dimensional scan patterns, the optical fiber and the illumination spot is typically driven in at resonant frequency of the optical fiber in a spiral shaped scan pattern. “Resonant” is used herein to mean that the optical fiber is driven within a Q-factor of its resonant frequency. When driven within the Q-factor of the resonant frequency (FIG. 3), the optical fiber can achieve its desired deflection while using a minimal amount of energy.
As shown in FIG. 1, in an idealized spiral scan pattern 11 the illumination spot typically starts at an initial, central position and spirals outward until a maximum desired diameter is reached. Once the illumination spot reaches its maximum diameter, it is desirable to return the illumination spot to the center. One proposed spiral scan pattern spirals the illumination spot outward to its maximum diameter, and then spirals the illumination sport inward by retracing the original spiral pattern. Of course, if desired, it may be possible to start the scan pattern at its maximum diameter and then spiral the illumination spot inward toward the middle.
To achieve the resonant spiral scan pattern 11 of FIG. 1, the optical fiber is driven along two drive axes (y and z or horizontal and vertical) with horizontal and vertical triangle amplitude modulated sine waves 13, 15 that are driven with a 90 degree phase shift between them (FIG. 2). If the optical fiber is circular, the horizontal and vertical resonant vibrations will have the same frequency and equal amplitude (but still 90 degrees out of phase). An increasing amplitude portion 17 of the drive signals 13, 15 cause the illumination spot to spiral outward from the initial, central position. The decreasing amplitude portion 19 of the drive signals 13, 15 cause the illumination spot to spiral inward, back toward the initial, central position. It was contemplated that images of the target area could be obtained by collecting back scattered light during the increasing amplitude portion 17, the decreasing amplitude portion 19, or both.
Applicants have found that there are two significant issues when driving a scanning element using the drive signals 13, 15 of FIG. 2.
First, because the scanning element is typically driven substantially in resonance, the scanning element tends to store a large amount of energy. The stored energy will cause the scanning element to continue to move at large amplitudes even after the amplitude of the drive signal is reduced (or removed). For example, in experiments Applicants have found that the illumination spot will follow the increasing amplitude portion 17 of the drive signal rather closely, but when the drive signal moves to the decreasing amplitude portion 19, the illumination spot does not follow as closely. Instead, the stored energy in the scanning element causes the illumination spot to decrease its scan diameter at a much lower rate than the drive signal.
While image and drive remapping methods (described in commonly owned and copending U.S. patent application Ser. No. 10/956,241, filed Oct. 1, 2004, the complete disclosure of which is incorporated herein by reference) can correct for the distortions in the image caused by the differences between the theoretical position of the illumination spot defined by the drive signal and the slower, actual position of the illumination spot, the stored energy in the resonating fiber may also cause a “hole” to appear in the center of the image (FIG. 4). Remapping methods can not correct for the hole in the center of the image. The hole in the center of the image is caused when the drive signal repeats the increasing amplitude portion of the drive signal before the illumination spot can return back to the center of the image (e.g., its initial, center position). In some scenarios, a diameter of the hole may be equal to half of the image, or more.
The second issue also involves the fact that the scan pattern may vary during the increasing amplitude portions and decreasing amplitude portions 17, 19 of the scan pattern 11. Using image or drive remapping and other techniques, however, it is possible to generate images in both scan directions that appear identical. Unfortunately, the actual scan pattern of the illumination spot may change depending on the environmental factors at the site of its use. Typically, temperature has the biggest effect on the scan pattern. For example, on the increasing amplitude portion 17, the changes caused by the higher or lower temperatures may be small and can be ignored or otherwise compensated for with the remapping methods. But for the decreasing amplitude portion 19, the changes caused by the temperature may be harder to correct. The temperature factors may cause images in the two scan directions to change in opposite ways. For example, on the increasing amplitude portion, the image may rotate clockwise, while during the decreasing amplitude portion the image may rotate counter-clockwise. If images are captured during both the increasing and decreasing amplitude portions, this may result in a display in which two diverging images are toggled at the frame update rate, which will cause the resultant captured image to become useless.
Therefore, what are needed are methods and systems which can provide high frame rates while accurately generating an image of the target area. It would be desirable if such methods and system can compensate for the energy stored in the resonating scanning element.